Finally, in proteins with more than one polypeptide chain, there is a fourth level of organization. Besides the interactions between amino acids within each chain, some such reactions occur between links in separate chains. These are never covalent reactions (as disulphide bonds do not form), but hydrogen and ionic bonding cause each polypeptide in the protein to tangle up with the others.

Proteins are polymers whose molecules are made from many amino acid molecules linked together. Proteins have a very wide range of different functions in living organisms. Some proteins, such as haemoglobin, enzymes and antibodies are involved in metabolic reactions, while others, such as collagen and keratin form the structure of living organisms. The function of any particular protein is related to its shape. Proteins themselves have four levels of structural organization. Each of these levels is associated with particular types and patterns of bonding, which combine to result in the overall shape of the protein.

There are twenty different, naturally occurring amino acids. amino acids can link to form polypeptide chains and these can associate with one another to form proteins. All amino acids contain a central carbon atom, known as Cα, to which four different groups are bonded. One of these is a hydrogen atom, which plays no part in the amino acid's function; the others are an amine group (-NH2), a carboxylic acid group (-COOH) and the "R-group". It is this variable R-group which is different between amino acids.

In fact, while amino acids are in solution they are not in this form: the proton from the carboxylic acid group reacts with the basic amine group, to form a -COO- group and a -NH3+ group. When an amino acid is in this form, it is called an internal salt or zwitterion.

Since the Cαcarbon has four different groups bonded to it, it can exist in two different three-dimensional spatial arrangements. This is known as optical isomerism or chirality. However, in nature only one of the forms, known as enantiomers, is ever found. This is because once one system had been chosen in a 'frozen accident' it was easiest for new molecules to fit the same pattern: any difference would be lethal, or at least very strongly selected against.

The twenty different amino acids which exist in nature can be combined in any combination to make a protein. However, this order is of tremendous significance as a change in just one amino acid can drastically change the behaviour of the protein formed. The number and sequence of amino acids in a polypeptide is known as its primary structure. The primary structure of a protein determines its overall shape and therefore its function.

The chain of amino acids which makes up a polypeptide chain does not remain perfectly straight but twists into a shape known as the secondary structure. This is determined by the strength and direction of hydrogen bonding within the polypeptide chain. The combination of these bonds results in the formation of one of two kinds of secondary structure: the α-helix and the β-pleated sheet.

In an α-helix, the chain twists into a regular spiral similar to a telephone cord. The helix is held together by hydrogen bonds between the (-NH) group of one amino acid and the (-CO) group of the amino acid four places ahead of it in the chain. Most proteins have at least part of their structure in the form of an α-helix.

In a β-pleated sheet, on the other hand, the chain is not tightly coiled, but lies almost straight. Often, several â strands lie side by side, and form hydrogen bonds with one another. Again, these bonds are between (-NH) groups and (-CO) groups but this time they are from different polypeptide chains. The result is a group of polypeptide chains which form a sheet. β-pleated sheets form part of the structure of most proteins.

The tertiary structure is the overall, three-dimensional structure of a polypeptide chain or protein. The amino acid chain, which is already in the form of an &alpha;-helix or β-pleated sheet, coils again to form a very precise shape which is characteristic of the specific protein. The shape is held firmly in place by bonds between amino acids which lie close to each other in the three dimensional structure. There are four types of bonds: van der Waals forces, hydrogen bonds, ionic bonds and disulphide bridges.

In some proteins, the tertiary structure forms a long, super-coiled chain, usually with a very regular repeating pattern. These proteins are called fibrous proteins. They are used for producing various structures in organisms such as keratin in nails, or fibrin, which causes blood clots to form.

In other proteins, the tertiary structure is more spherical, forming a globular protein. Globular proteins are usually soluble and are involved in metabolic reactions inside and outside cells. Some examples include haemoglobin, enzymes and some hormones.

Finally, in proteins with more than one polypeptide chain, there is a fourth level of organization. In proteins such as insulin, hydrogen and ionic bonding cause two or more polypeptide chains curl together to form a complete protein molecule. This is known as the quaternary structure of the protein. The different polypeptide chains are held together by the same types of bonds which are responsible for the tertiary structure.

In chemical analysis, the total nitrogenous material in vegetable or animal substances, obtained by multiplying the total nitrogen found by a factor, usually 6.25, assuming most proteids to contain approximately 16 per cent of nitrogen.